National Renewable Energy Laboratory Innovation for Our Energy Future

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CIGSS Thin Film Solar Cells Final Subcontract Report 11 October 2001—30 June 2005 N.G. Dhere Florida Solar Energy Center Cocoa, Florida

Subcontract Report NREL/SR-520-39486 February 2006

CIGSS Thin Film Solar Cells Final Subcontract Report 11 October 2001—30 June 2005 N.G. Dhere Florida Solar Energy Center Cocoa, Florida

NREL Technical Monitor: B. von Roedern Prepared under Subcontract No. NDJ-2-30630-03

Subcontract Report NREL/SR-520-39486 February 2006

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Table of Contents

Introduction..........................................................................................................................1 Selenization/Sulfurization Furnace......................................................................................2 Heterojunction Partner Layer, CBD CdS...........................................................................10 Rapid Thermal Processing (RTP) ......................................................................................11 References..........................................................................................................................16 List of Figures Figure 1: SEM micrograph .................................................................................................3 Figure 2: SEM micrograph of Sample 2 .............................................................................4 Figure 3: SEM micrograph of Sample 3 .............................................................................5 Figure 4a: Se/S treated and CdS deposited .........................................................................5 Figure 4b: AES depth profile..............................................................................................5 Figure 5: AES elemental depth profile ...............................................................................6 Figure 6: SEM of sample grown in Cu-rich composition...................................................7 Figure 7: SEM of sample with dwell of 140°C for 30 minutes and ramp

of 20°C/minute ....................................................................................................7 Figure 8a: SEM...................................................................................................................8 Figure 8b: As deposited and Se treated...............................................................................8 Figure 9a: 1- as deposited, 2- selenized and CdS treated, 3- selenized ..............................9 Figure 9b: SEM of absorber layer.......................................................................................9 Figure 10a: Se/S treated ......................................................................................................9 Figure 10b: SEM image ......................................................................................................9 Figure 10c: Se/S treated sample 10b.................................................................................10 Figure 10d: SEM of sample 10b .......................................................................................10 Figure 11: RTP System Configuration .............................................................................12 Figure 12: RTP Schematic ................................................................................................12 Figure 13: Plot of temperature vs. time at 100% power ...................................................14 Figure 14: Plot of temperature vs. time 30%, 40%, and 0% power..................................14 Figure 15: Plot of temperature vs. time for power fixed at 35% (constant) .....................14 Figure 16: Mechanical drawing for custom-made graphite tray.......................................15 Figure 17: RTP setup ........................................................................................................15 Figure 18: RTP setup during actual run............................................................................15

iii

iv

Introduction

I-III-VI2 compounds are developing into a promising material to meet the energy

requirement of the world. CuInSe2 (CIS) and its alloy with Ga and S have shown long

term stability and highest conversion efficiency of 19.5% [1]. Among the various ways of

preparing CuIn1-xGaxSe2-ySy (CIGSS)/CdS thin-film solar cells, co-evaporation and

sputtering techniques are the most promising. Sputtering is an established process for

very high-throughput manufacturing. ARCO Solar, now Shell Solar pioneered the work

in CIS using sputtering technique [2,3]. The two stage process developed by ARCO Solar

involved sputtering of a copper and indium layer on molybdenum coated glass as the first

step. In the second step, the copper-indium layers were exposed to a selenium-bearing

gas such as hydrogen selenide (H Se) mixed with argon. The hydrogen selenide breaks

down and leaves selenium, which reacts and mixes with the copper and indium in such a

way to produce very high-quality CIS absorber layer. Sputtering technology has the

added advantage of being easily scaled up and promotes roll to roll production on flexible

substrates. In the early 90’s, a nontoxic selenization process was developed at the PV

Materials Lab to avoid use of extremely toxic H Se gas. The two stage selenization

process involved deposition of copper-indium layer with excess copper. The elemental

stack was selenized by heating the substrate in the presence of selenium vapors obtained

by thermally evaporating elemental selenium. Selenization of the copper-rich film helped

improve adhesion with molybdenum back contact. The copper-rich film was further

deposited with indium and re-selenized to produce an efficient thin-film CIS absorber [4].

The process was further modified and gallium concentration was optimized to produce

cell with efficiency 9.02% [5]. As of now, CIGSS/CdS thin film solar cells are being

prepared by rapid thermal processing and conventional selenization using diethylselenide

(DESe) as selenium source [6,7] and H S as sulfur source [8], in combination with

DC/RF magnetron sputtering and chemical bath deposition technique. Sulfurization of

metallic precursors is a well-developed process to produce a high bandgap (1.55eV)

absorber. Hahn-Meitner-lnstitut of Germany has developed a similar process using

elemental sulfur [9]. A new selenization set-up for DESe was build and moved near the

furnace to reduce the distance between the furnace and selenium source. Preliminary

2

2

2

1

experiments for selenization were carried out. X-ray diffraction and electron probe

microanalysis have shown promising results. Experiments were carried out to establish

optimum parameters for CuGa-In precursor deposition as well as

selenization/sulfurization. A rapid thermal processing set-up was assembled. Preliminary

experiments were carried out. ZnO/ZnO:Al deposition by RF magnetron sputtering and

CdS deposition by chemical bath deposition (CBD) are being carried out on routine basis.

CdS setup was modified to reduce the consumption of the solution as well as to heat the

chemical bath uniformly [10].

Selenization/Sulfurization Furnace

Three new temperature controllers from Eurotherm were procured. The necessary

enclosure was procured and all three controllers along with an over-temperature

controller have been properly installed in the enclosure while the enclosure itself was

installed securely beneath the furnace. The controllers were connected to the

selenization/sulfurization furnace and are being used to control the heat treatment process

for the development of CIGSS thin films on glass and stainless steel substrates. The

controllers were programmed with the requisite parameters and initially tested for

time/temperature readings. The new temperature controller has reduced the temperature

fluctuation during the process. A temperature ramp of 6 oC/ minute is being routinely

used for selenization/sulfurization of the elemental stack because it led to the minimum

error between the set values and the actual values. Experiments were carried out to

establish selenization cycles in order to achieve complete selenization of the elemental

stack. A series of experiments was performed for optimizing the heat treatment

parameters such as temperature and soaking time to ensure complete selenization and

formation of a device quality absorber.

Metallic precursors CuGa and indium were sputter-deposited on molybdenum

coated glass substrate. Power and pressure for precursor deposition was kept the same in

all experiments discussed here. The speed at which the substrates moved over the targets

of each precursor layer was varied in order to obtain desired rates of depositions, total

thicknesses of each layer and consequently the stoichiometric composition. As mentioned

2

in the Year 3 Annual Report, the substrate movement speed was controlled with a PC-

controlled stepper motor using a LABVIEW program.

Experiment #1- To set initial parameters for selenization and sulfurization.

Metallic precursor - CuGa was deposited at linear speed of 0.048 cm/sec while

indium was deposited at a linear speed of 0.021 cm/sec. The elemental stack was

selenized at 400oC for 10 minutes and sulfurized at 475oC/20 minute in a single run.

i:ZnO and ZnO:Al window bilayers were deposited using RF magnetron sputtering at 200

Watt at 0.048 cm/sec and 300 Watts at linear substrate movement speed of 0.006 cm/sec.

Ni/Al contact fingers were deposited using a metal mask by electron beam evaporation.

EPMA: sample 1

Elem. Cu In Ga Se S

At% 26.57 25.64 3.78 17.11 26.88

Figure 1: SEM micrograph

From SEM analysis (Fig. 1) it was seen that the grain size was very small and the

grains were not faceted. EPMA analysis was carried out to check the stoichiometry and it

was observed that the amount of selenium was comparatively lower than the sulfur

amount. This indicated that there was not enough Se in the atmosphere or the temperature

and soaking time were not adequate for completion of the reaction between metallic

precursors and Se. Initially sulfur reacts with the unreacted Cu-Ga-In. It can also replace

selenium because it is more reactive than selenium.

Sulfurization is considered as a bandgap engineering treatment. The open-circuit

voltage Voc mainly depends on the band gap in the space charge region. CIGS has a

bandgap of 1.12 eV with Ga ~ 30 at% resulting in lower Voc. Post sulfurization treatment

leads to the formation of CIGSS or CuIn1-xGaxS2 layer with a wider bandgap of 1.55eV at

the p-n junctions that helps in increasing the open circuit voltage (Voc). During post

3

sulfurization treatment the sulfur atoms occupies already existing selenium vacancies,

thereby reducing the compensating donor and also passivating the surface [11].

EPMA:Sample 2a with lower sheet resistance

Elem. Cu In Ga Se S

At% 31.27 22.79 3.73 15.16 27.03

EPMA:Sample 2b with higher sheet resistance

Elem. Cu In Ga Se S

At% 27.87 20.25 3.34 13.82 34.70

Figure 2: SEM micrograph of Sample 2

Experiment #2-The purpose of this experiment was to see if selenium incorporation

increases in presence of sodium and excess copper.

Addition of controlled amount of sodium helps in growth kinetics of the

chalcopyrite film resulting in crystalline film [12]. Two changes were made in this

experiment. First the amount of CuGa was increased by reducing the substrate movement

speed from 0.048 cm/sec to 0.04 cm/sec. Secondly small amount of sodium was added in

the form of NaF layer. 3.4 mg of NaF was thermally evaporated onto the Mo surface.

NaF melted and then evaporated at a current of 160-165 A. Calculations indicated that

this amount corresponds to 60 to 120 Å. Some part of the film peeled off at Mo/CIGSS

interface. Peeling was due to improper cleaning and probably due to NaF layer at the

interface. The film was patchy, a part of it had sheet resistance of 250 Ω/ while the other

had 300 Ω/. Sample 2a was from the lower sheet resistance region indicating more

copper while Sample 2b was from the comparatively higher sheet resistance. Overall the

film was copper-rich as Cu/In+Ga ration was > 1. The problem of incomplete

selenization continued as the amount of Se in the films was consistently low.

4

Experiment #3 – To check the consistency of results in Experiment 2

le 3a EPMA: Samp

EPMA: Sample 3b

Elem. Cu In Ga Se S

At% 26.86 24.4 3.7 13.69 31.33

Elem. Cu In Ga Se S

At% 27.94 21.52 3.53 12.85 34.14

Figure 3: SEM micrograph of Sample 3

A thin layer of NaF was deposited on the precursor layer instead of Mo layer as it

was believed to be one of the potential reasons for peeling observed in Experiment 2. The

film appeared uniform after the selenization cycle. SEM micrograph (Fig. 3) revealed

well-developed, compact and distinct grains with sizes varying from 0.5 µm to 2 µm. It

was again evident from EPMA analysis that the proportion of selenium was considerably

lower than that of sulfur. After CdS chemical bath deposition on the absorber layer, a

patchy morphology that was not discernible on earlier selenized/sulfurized films

appeared. Another important observation was no peeling-off of the absorber film.

0

10

20

30

40

50

60

0 5000 10000 15000 20000 25000 30000

Depth (Ang.)

Con

cent

ratio

n at

. %

Cu In Ga S O C Mo Se

Se Ga

In

S

Cu

Figure 4a: Se/S treated and CdS deposited Figure 4b: AES depth profile

Experiment # 4 – To increase Se incorporation by supplying more selenium-containing

vapor.

Some experiments were carried out at a higher Se pressure and a constant heating

rate of 6oC/min so as to overcome selenium deficiency. Selenization was carried out at

5

400oC for 15 minutes so as to increase Se incorporation in the films. Selenization was

followed by sulfurization at 475oC for 15 minutes. Sulfurization time was reduced as the

intention was to have more Se than S in the films. Diluted DESe pressure was increased

from earlier value of 30 Torr to 60 Torr in order to increase the amount of Se. The heat-

treated film showed three patches with sheet resistance of 700 Ω/, 1000 Ω/ and 1200

Ω/. Half of the sample was etched in 10% KCN and subsequently oxidized in equal

quantities of H2O2 and 1% H2SO4. The remaining half was used as selenized/sulfurized

without any surface treatment. Both the pieces were coated with CdS layer. As shown in

the Figure 4a, the etched oxidized sample appears bluish purple while the other appears

gray with three patches clearly visible. Though EPMA data was not available, it was

observed from AES data that selenium incorporation was not significant.

Experiment #5 – To prevent indium loss if any, during selenization

An additional dwell was incorporated at 140oC for 30 minutes to prevent indium

loss during selenization. Selenization and sulfurization were carried out with similar ramp

of 6oC/min with temperature and dwell of 400oC for 15 minutes and 475oC for 15 minute

respectively. Total pressure of DESe diluted in hydrogen was 60 Torr. During chemical

bath deposition excessive thickness of CdS was deposited. The film showed yellow CdS

patches all over the film. Also from AES it was observed that the amount of Se was less

than that of S. Elemental depth profile of the absorber layer grown with these parameters

is shown in Figure 5.

0

10

20

30

40

50

60

70

80

90

0 5000 10000 15000 20000 25000 30000

Depth (Ang.)

Con

cent

ratio

n at

. %

Cu In Ga S O C Mo Se

Se

In S

Cu

Figure 5: AES elemental depth profile

6

Experiment # 6 – To study the effect of excess copper

From the AES depth profile shown in Figure 5, it was seen that the amount of Se

in the absorber film was very low even after increasing the amount of diluted DESe (from

30 Torr to 60 Torr). Therefore, selenization was carried out on the excess copper film.

Indium layer deposited at substrate speed of 0.04329 cm/sec was sandwiched between

two layers of CuGa deposited at substrate speeds of 0.06 and 0.138 cm/sec respectively.

No NaF was added as the film had excess Cu. SEM micrograph is shown in Figure 6.. In

all the experiments carried out so far, Se incorporation was found to be very less. Hence,

further experiments were carried out for preparing selenide absorber layer.

EDS of etched film grown in Cu-rich composition

Elem. Cu In Ga Se

At% 22.16 22.16 5.05 49.58

Figure 6: SEM of sample grown in Cu-rich composition

EDS: sample with additional dwell and higher ramp

Elem. Cu In Ga Se

At% 22.86 24.93 3.16 47.82

Figure 7: SEM of sample with dwell of 140oC for 30 minutes and ramp of 20oC/minute

Experiment # 7- To study the effect of higher heating rate and dwell at lower temperature

In the following experiment, the deposition parameter and the maximum

temperature for selenization were similar to the previous experiment, except that in this

case a heating ramp of 20oC/minute and an additional dwell at 140oC for 30 minutes was

7

used. CdS, i:ZnO/ZnO:Al and Ni/Al contact fingers were deposited. During the

measurement, it was realized that the Mo layer from absorber layer interface to the

substrate had completely reacted even under the Cu_In-Ga layers, leaving no Mo back

contact for efficiency measurement. The thickness of these films was measured using a

thickness profilometer and was found to be 1.3 to 1.5 µm. This thickness was almost half

of the value intended (2.75 µm). SEM of the absorber layer is shown in Figure 7.

As deposited selenized

Figure 8a: SEM Figure 8b: As deposited and Se treated

EDS: from light gray patch EDS: from murky gray patch

Elem. Cu In Ga Se

At% 21.25 22.71 4.24 50.54

Elem. Cu In Ga Se

At% 22.49 21.19 5.38 49.13

Experiment #8- To optimize selenization time at maximum selenization temperature

This sample (Figure 8b) had a layer of NaF on top of metallic precursor layer. The

sample was selenized at 400oC for 10 minutes. Diluted DESe pressure was maintained at

60 Torr. The sample showed a dark gray patch as seen in Figure 8b. The sheet resistance

in this region was 40 - 60 Ω/ and 150 to 200 Ω/ in the remaining uniform gray region.

The dark patch was believed to be that of non-reacted film. So the dwell time was

increased to 15 minutes in the next experiment.

Another sample shown in Figure 9a-3 was selenized at 400oC/15 minutes and it

showed a pattern after taking out from the furnace. The pattern remained after etching.

The entire film turned dark on oxidation. The film also peeled off during CdS deposition

(Figure 9a-2). SEM of the film is shown in Figure 9b. In both the films shown in Figure

8a and 9b, the thickness of the absorber layer was in the range of 1.3 to 1.5 µm. Due to

8

the lower thickness, Mo layer was completely reacted, as a result of which devices could

not be prepared using the film.

Deposited selenizedCdS

treated

3

2 1

Figure 9a: 1- as deposited, 2- selenized Figure 9b: SEM of absorber layer

and CdS treated, 3- selenized

Experiment # 9- Thickness of the absorber was doubled to prevent complete Mo reaction

In next experiment, the deposition timing was doubled so as to obtain higher

thickness of the absorber film. As mentioned earlier, the thicknesses were in the range of

1.3 -1.5 µm so the deposition timing was doubled. CuGa was deposited in two layers; the

first layer was deposited at a substrate speed of 0.03 cm/sec while the second layer was

deposited at 0.07 cm/sec. An indium layer deposited at a substrate speed of 0.021 cm/sec

was sandwiched between the two CuGa layers.

Figure 10a: Se/S treated Figure 10b: SEM image

A 5 cm x 10 cm strip from this deposition was selenized at 400oC for 10 min and

sulfurized at 475oC for 10 minute. 1% of DESe was used for selenization. The partial

pressure of diluted DESe was 30 Torr. The temperature was ramped up at 20oC/min with

9

no intermediate dwell. The sample showed patches of dark and light gray area as seen in

Figure 10a. Sheet resistance in the dark areas was 1.4 kΩ/ and 1 kΩ/ in the light gray

areas. Overall the film was slightly copper-rich which was evident from the CuSe

whiskers seen in the SEM micrograph (figure 10 b).

Experiment # 10- To study the effect of reduced ramp rate

In another experiment, the ramp rate was reduced to 6oC/minute. This sample

showed a patch too as seen in Figure 10c. However, the film showed overall uniformity

compared to the film shown in Figure 10a. The sheet resistance was 300 to 500 Ω/ and

the thickness was 3.8 to 3.9 µm. SEM image is shown in Figure 10d.

Figure 10c: Se/S treated sample 10b Figure 10d: SEM of sample 10b

The grain size varied from <1 µm to 2 µm in all the samples. Sulfur quantity was

higher in all the films. Cells were completed and efficiency was measured. The values

obtained were not very attractive and it was inferred that the process needs to be modified

to optimize the parameters to obtain good quality absorber.

Heterojunction Partner Layer, CBD CdS

Heterojunction partner CdS layers were satisfactorily deposited by chemical bath

deposition on 10 x 10 cm2 CIGS2 samples. The CdS deposition set-up was modified to

reduce the quantity as well as to ensure uniform heating of the solution. A 4000 ml

beaker was used as a water bath while the reaction solution was prepared in a 1000 ml

10

beaker. Two samples of size 5 cm x 10 cm could be coated with a conformal layer of CdS

in a single run.

Rapid thermal processing (RTP)

A Rapid Thermal Processing (RTP) unit has been designed, constructed and

installed for preparation of CIGSS thin films on 10 cm x 15 cm substrates by

selenization/ sulfurization of elemental precursors using the vacuum deposited selenium

layer and N2:H2S atmosphere. For RTP, selenium evaporation will be carried out by

thermal evaporation in a separate setup. An optimized, minute amount of NaF was

deposited to improve the morphological and electrical properties of the absorber.

The configuration and schematic are shown in Fig. 11 and 12 respectively. A

quartz tube (ID = 15 cm) was mounted with a stainless steel flange assembly.

Feedthroughs were mounted on the end flange for connection to a thermocouple which

would monitor temperature. Viton rubber ‘O’ ring in a groove was used to form an air-

tight seal between the hard surfaces of the flange and glass tube. The flange had two

openings – a gas inlet (for H2S and Se vapors) and an exhaust. A pair of high density

infrared heaters (Research Inc., Model 5090), each with a bank of 5 T3-style quartz

infrared halogen lamps directs energy onto the reaction tube where the substrate was

kept. Each lamp is rated at 1200 Watts. The power to the lamps was controlled by a pair

of single-phase (120V AC, 55 A) standalone SCR power controls (Research Inc., Model

5620). Each T3-style quartz halogen lamp radiates 90% energy in less than 1 second.

The two heater arrays have to be controlled independently so that the thermal

gradient across the thickness of the substrate remains symmetrical during all RTP

processing phases. This prevented the substrate from breaking and cracking.

11

lamp

reactor tube

substrate

substrate holder

reflector

Figure 11: RTP System Configuration

Infrared Heater with electrical and water connections

Gas inlet

Figure 12: RTP Schematic

Each heater consists of a specular aluminum reflector that directs the infrared energy

supplied by the lamps on to the substrate. Connections to supply required cooling water

to the heater were provided. The heaters were mounted on a stand so as to face both the

surfaces of the substrate. Two aluminum sheets with 95% reflectivity and measuring 30

cm x 30 cm x 0.05 cm each were formed into arcs and installed laterally on both sides of

the quartz tube to reflect the heat back to the substrate during RTP. Circular reflective

stainless steel sheets were placed within the quartz tube at both ends. A sliding

Lid

Array of 5 Tungsten Halogen Lamps

Reaction Tube (Quartz)

Thermocouple

Substrate to be processed

Mobile mounting frame

Sliding wheel

Flange assembly

Exhaust Valve

To mechanical pump

12

mechanism was provided to quickly move the heaters away from the reaction tube after

the completion of the process for rapid cooling.

Experiments were carried out in order to determine the rate at which substrate

temperatures would rise with the current setup. A mechanical pump was used to achieve

a vacuum in the range of 10-3 Torr within the quartz tube. Cool water was circulated

through each of the two infrared heaters at a rate of approximately 1.4 liter per minute per

heater. The two power controllers for the infrared lamp arrays were switched to ‘operate’

mode. The power was raised to 100%. The temperature of the substrate was measured

by means of a K-type chromel-alumel thermocouple. An initial temperature ramp of >6 oC per second and an average temperature ramp of 3.96 oC per second were obtained so

that the temperature increase from 25 oC to 500 oC could be attained in 120 seconds at

100% power (Figure 13). The control power was slowly reduced and the power supply

was switched off after reaching the temperature of 500 °C.

The next set of experiments was carried out to determine the fixed power to the

two heaters for slowly reaching and maintaining a temperature of 500 oC. These initial

experiments were carried out to model the behavior of the RTP setup with respect to the

temperature-time profile obtained for a particular power setting. The ramps thus obtained

were not as high as those required in RTP. Initially, the power was kept at 30% of the

maximum. The temperature rose to 430 oC in 48 minutes and then remained stable. The

power was then increased to 40%. Temperature rose to 500 oC in 2 minutes with a

tendency to rise further. At this stage the power was switched off (Figure 14). In the

subsequent experiment, it was found that when total lamp power was fixed at 35%, the

substrate temperature rose to 500 oC in 52 minutes and remained stable (Figure 15). The

results of these experiments are being used to maintain a given temperature during the

desired interval after reaching that temperature at a very high rate using 100% power.

13

The following graphs show ramp rates for various power settings:

Temperature vs. time

0

100

200

300

400

500

600

12 32 57 89 120

Time (seconds)

Tem

pera

ture

(deg

. C)

100%

Figure 13: Plot of Temperature vs. time at 100% power

Temperature vs Time

0

100

200

300

400

500

600

0

2919

3047

3060

3187

3263

3610

4112

4598

Time (seconds)

Tem

pera

ture

(deg

. C)

30%, 40%, OFF

off

30%

40 %

Figure 14: Plot of temperature vs. time for 30%, 40% and 0% power

Temperature vs time

0

100

200

300

400

500

600

Time (seconds)

Tem

pera

ture

(deg

. C)

35%

Figure 15: Plot of temperature vs. time for power fixed at 35% (constant)

Towards the end of the quarter, a high-purity, pyrolytic graphite custom-made

tray (Fig. 16) was procured from Poco Graphite to hold a 10 cm x 10 cm sample at the

center of the rapid thermal processing (RTP) quartz chamber. The graphite tray was

designed in such a way that the sample could be kept in the tray. The graphite tray would

rest on a larger quartz plate of size 14 cm x14 cm x 0.31 cm.

14

Figure 16: Mechanical drawing for custom-made graphite tray

Process gas inlet

Chromel-alumel thermocouple

QF-25 clamp

QF-40 clamp

Graphite tray

Mechanical pump inlet

Figure 17: RTP Setup Figure 18: RTP setup during

actual run

15

References

[1] M. A. Contreras, K. Ramanathan, J. AbuShama, F. Hasoon, D.L. Young, B. Egaas,

and R. Noufi, Progress in Photovoltaic: Research and Application, 13, 209, (2005)

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[8] N.G. Dhere, S.R. Ghongadi, M.B. Pandit, A.H. Jahagirdar, and D. Scheiman, Progress

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[9] R. Klenk, U. Blieske, V. Dieterle, K. Ellmer, S. Fiechter, I. Hengel, A. J~iger-

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[10] N. G. Dhere, S. S. Kulkarni and A. A. Kadam, “CdS Bath Recycling and Toxic

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[11] T. Nakada, H. Ohbo, T. Watanabe, H. Nakazawa, M. Matsui, A. Kunioka, Solar

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February 2006 2. REPORT TYPE

Subcontract Report 3. DATES COVERED (From - To) 11 October 2001 – 30 June 2005

5a. CONTRACT NUMBER DE-AC36-99-GO10337

5b. GRANT NUMBER

4. TITLE AND SUBTITLE CIGSS Thin Film Solar Cells: Final Subcontract Report, 11 October 2001 – 30 June 2005

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER NREL/SR-520-39486

5e. TASK NUMBER PVB65101

6. AUTHOR(S) N.G. Dhere

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Florida Solar Energy Center 1679 Clearlake Road Cocoa, FL 32922-5703

8. PERFORMING ORGANIZATION REPORT NUMBER NDJ-2-30630-03

10. SPONSOR/MONITOR'S ACRONYM(S) NREL

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401-3393

11. SPONSORING/MONITORING AGENCY REPORT NUMBER NREL/SR-520-39486

12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161

13. SUPPLEMENTARY NOTES NREL Technical Monitor: B. von Roedern

14. ABSTRACT (Maximum 200 Words) This report describes the I-III-VI2 compounds that are developing into a promising material to meet the energy requirement of the world. CuInSe2 (CIS) and its alloy with Ga and S have shown long-term stability and highest conversion efficiency of 19.5%. Among the various ways of preparing CuIn1-xGaxSe2-ySy (CIGSS)/CdS thin-film solar cells, co-evaporation and sputtering techniques are the most promising. Sputtering is an established process for very high-throughput manufacturing. ARCO Solar, now Shell Solar, pioneered the work in CIS using the sputtering technique. The two-stage process developed by ARCO Solar involved sputtering of a copper and indium layer on molybdenum-coated glass as the first step. In the second step, the copper-indium layers were exposed to a selenium-bearing gas such as hydrogen selenide (H Se) mixed with argon. The hydrogen selenide breaks down and leaves selenium, which reacts and mixes with the copper and indium in such a way to produce very high-quality CIS absorber layer. Sputtering technology has the added advantage of being easily scaled up and promotes roll-to-roll production on flexible substrates. Preliminary experiments were carried out. ZnO/ZnO:Al deposition by RF magnetron sputtering and CdS deposition by chemical-bath deposition are being carried out on a routine basis.

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15. SUBJECT TERMS

PV; thin film; solar cells; CuInSe2 (CIS); CuIn1-xGaxSe2-ySy (CIGSS); co-evaporation; sputtering; chemical bath deposition (CBD);

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Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18